The invention relates to a positive-action light-sensitive mixture comprising a water-insoluble resinous binder, which is soluble or swellable in aqueous alkaline solutions, and a light-sensitive 1,2-naphthoquinone-diazidesulfonic acid ester of a trihydroxy-phenylketone. Such compositions are particularly suitable for production of planographic printing plates.
Light-sensitive compounds of this general type are disclosed in German Pat. No. 938,233 and U.S. Pat. No. 3,802,885. They are dihydroxybenzophenones or trihydroxybenzophenones which are partially or completely esterified with naphthoquinonediazidesulfonic acids. Recently, the art has evidenced a preference for completely esterified compounds, which no longer contain any free phenolic OH groups. These compounds have relatively high light sensitivity and give long printing runs, but they have the disadvantage that for some purposes their solubility in the organic solvents usually employed for the coating of printing plates, such as glycol partial ethers or carboxylic acid alkyl esters (e.g. butyl acetate), is too low. Furthermore, used in combination with the preferred binders, for example novolaks, which are soluble in aqueous alkaline solvents, they give layers which in the unexposed state are not sufficiently resistant to conventional alkaline developer solutions. These layers are also attacked to a certain extent during printing in offset printing machines, if alcohol-containing fountain solutions are used. Moreover, they are sensitive to gasoline hydrocarbons frequently used during printing to wash the oleophilic printing ink off the plate.
German Pat. No. 1,118,606 discloses naphthoquinone-diazidesulfonic acid esters of phenolic compounds having a somewhat different basic structure, for example esters of diphenylmethane derivatives. These esters also always contain at least one free hydroxyl group.